In February, my bank account was hacked into and cleared out of £14,000. My bank, Santander, paid me back without a problem. However, on the same day my Orange mobile phone stopped working. I got a new sim card from Orange, and thought no more about it. My mobile number was on my bank records that were intercepted online.

Now Orange has demanded £315 for mobile phone usage on those days – calls to Sierra Leone and Algeria, as well as others. They insist that the calls were made from my sim card.

On the days in question I was at home (I am retired). The phone is only accessible to myself and my husband, who doesn't even know how to use it, let alone make calls to Africa.

The Orange operator I spoke to was rude, insisting I must have made the calls. When I asked for a supervisor, he hung up. I called again, getting a supervisor after some argument with the operator. I explained all and he said he would call me back, but never did. I called again, and another terse operator told me it was not possible to speak to a supervisor.

He said an "investigation" had found I had made the calls, and I was made to feel that I was lying. I have consulted with an IT "whizz kid", who tells me that the coincidence of the money going, telephone number being accessed, and the calls being made around the same date, was too strong to ignore.

I have written to Orange, but have heard nothing. If I do not pay, I will get a bad credit rating. AP,Hatfield, Herts

Yours is a worrying letter for anyone who does online banking and receives notifications of codes required to move money to their mobile. It appears that your bank account was taken over.

To clear out your account, the thieves then took over your mobile phone account, too.

Santander is one of several banks that sends its customers a special code when they want to move money to an account outside the bank. Get hold of the phone account, and you can then carry on with your fraud.

Santander has quickly done the right thing and paid you back in full. It told you that this was a particularly sophisticated fraud.

However, the same cannot be said of Orange. The fact that its staff refused to accept that you had been the victim of a fraud is, sadly, not unexpected.

One of the many problems with the mobile sector is that security is still lax. Many firms simply ask for your date of birth or your mother's maiden name – details that are easily available, or easy to find out. Armed with this it is simple to take over an account.

CIFAS, the UK's fraud prevention service, says anecdotal reports suggests more cases like yours are happening. It says that there were 6,136 proven mobile account takeover frauds last year, although the true number may be much higher. These takeovers tend to be carried out in conjunction with other attempts to defraud businesses and individuals, it warns.

We sent your case to Orange which now accepts that you didn't make the calls, and is sending you a full refund.

"Our initial research into this account suggests the customer has been a victim of fraud. However, we will be launching an internal investigation to establish what has happened in this case. We are very sorry for the experience the customer has received when reporting these issues and would like to apologise for any inconvenience caused," it says.

We welcome letters but cannot answer individually. Email us at consumer.champions@guardian.co.uk or write to Bachelor & Brignall, Money, the Guardian, 90 York Way, London N1 9GU. Please include a daytime phone number